Problem is tough, that there are many steps in between your current version and 9.601-5. Did you perhaps just install the system or do you have any firewalls in front of the UTM that might block it's own check for update feature?

You could also make a backup of your config, download the latest ISO, install that (it's an install from scratch) and then recover from your backup to have the config back. But there might be other ways to try first. Maybe even just try rebooting the firewall and see if that brings a change.....

What also may help is to stop automatic downloading of firmware files, clean folder again and then manually download a couple of update files (of course going from your current fw to the next, and the next and the next) from ftp-site and apply those. That way not all space for all update files is being used and only a couple of these are there. Because you only have a small amount of space too less, that might just do the trick.

So manually download and apply 3 or 4 updates and then have the UTM check for updates again by itself where it will then find the remaining updatefiles. With a little luck that will leave enough space to get the rest going in one go using webadmin.

Otherwise you need to find out how to free up space from your partition or start over from scratch (do not forget to first make a backup that can be read back after reapplying a new install from ISO).